A320 Fcom 1.22.30 P12 Speed/mach Switching

[Microburst2002]

I confess I don't fully understand when and why the selected speed target switches to mach target (or the opposite in descent).

When speed is managed I understand that, as there is a known speed/mach schedule, the FMGC knows what the crossover altitude is, and changes over exactly at the crossover altitude. Am I correct?

In selected speed, however, the FMGC doesn't know what our plans are for the climb (or descent), so it switches to mach (or speed) at a FL according to the table in this paragraph. Am I correct? For a given climb IAS there is a given mach number and a crossover FL? And for a given descend mach number there is a given IAS and a given crossover FL? If so, I don't know very well how to read that table.

For instance, climbing at 300 kt, What will occur? switch to 0.76 at FL265 always?
And what about descending at 0.76? will the target switch to 280 kt at FL305always?
What about climb speeds lower than 280? Is it that at any speed lower than that the switching will occur always at FL305, whatever the actual mach number?
And what about descending mach numbers lower that 0.76? Is it that at any mach lower than that the switching will occur always at FL 305 no matter what the IAS?

thanks

[rudderrudderrat]

Hi, I've found a similar table and agree with your changeover assumptions. The table does not seem to be complete. I'm assuming the "Crossed Box" is missing at FL315 and above the 280 kts /.76 column.

I'll investigate your .75 descent changeover FL and your 270 kts climb next chance I get.

I find Airbus is poor at providing information for pilots, it's almost as if they attempted to design the pilot out of the flight deck.

Rgds rudderrudderrat

[TyroPicard]

Had you bothered to search you would have found this...

http://www.pprune.org/tech-log/374966-a320-speed-mach-switching.html

[Microburst2002]

Thank you

However, they didn't come to a conclusion.

Still a "mistery"

[Microburst2002]

Rudderrudderrat

thank you

Let's see what empyrical research shows...

:)

[TyroPicard]

M2002
I confess I don't fully understand when and why the selected speed target switches to mach target
Do you need to understand? Why not just fly the aeroplane? You have chosen to use SEL SPD for a reason - just monitor the a/c and if you don't like the IAS or M after changeover then adjust it!
What difference would it make if there was no table?

[Microburst2002]

Yes, of course.
Climb and cruise mach number is the same most of the times for a given company CI, so all we have to do is switch the reference to Mach number manually when this particular mach number is reached, or switch back to speed if the FMGC switches it on its own before reaching it.

It is only I would like to know exactly how the system works. I don't like seeing a table in the FCOM that I don't know how to interpret.

[aristoclis]

What difference would it make if there was no table?


Then why does Airbus bother presenting a table in the FCOM which makes no difference?